Please note that Xubuntu comes with very limited multimedia support. The reason for this is that many popular multimedia formats are not free; they are protected by patent or license restrictions. To prevent problems, Xubuntu ships without support for these formats. Please consider using and supporting open and free multimedia formats such as Ogg Vorbis (Audio) and Ogg Theora (Video).

If, however, you still want to use restricted multimedia formats, you will need to enable the Universe and Multiverse repositories (see the section called “Extra Repositories”). You will want to install at least the following packages:

libxine-extracodecs 
ffmpeg 
lame 
faad 
sox 
mjpegtools 
libxine-main1
gxineplugin

With these packages installed, you should be able to play many multimedia formats. Please refer to https://wiki.ubuntu.com/RestrictedFormats for detailed information on getting even more formats to play.